Influence of pH on the Dissimilation of Glucose by Aerobacter indologenes

Abstract

A critical pH level exists in the dissimilation of glucose by A. indologenes in the region of pH 6.3. Fermentation carried out above this level results in an accumulation of acetic and formic acid. The production of H2 and C02 is greatly suppressed and the formation of acetylmethylcarbinol and 2,3-butylene glycol may be prevented if care is taken not to allow the pH to drop to around 6.3. When the fermentation occurs below pH 6.3, the acetic acid is converted into acetylmethylcarbinol and 2,3-butylene glycol. If sufficient acetic acid is added to an acid fermentation, the production of H2 is prevented. The acidity and alkalinity of the medium may perhaps express itself by determining the relative H-accepting ability of acetic acid and CO2. When the medium is alkaline, CO2 is the better H aceptor, resulting in the formation of formic acid; under acid conditions (approx. 6.3 or less) CO2 cannot compete with acetic acid and acetylmethylcarbinol and 2,3-butylene glycol are formed. For the conversion of acetic acid into neutral compounds, it is important to have both free acetic acid and available H. Evidence suggests that the general occurrence of formic acid among the final products of bacterial dissimilation of glucose may be the result of a synthesis from CO2 and H2, a fact which would have important implications in formulating schemes of dissimilation particularly with reference to the break-down of pyruvic acid into formic and acetic acids or acetalde-hyde and CO2. The results point clearly to the fact that the gas ratio of (H2CO2) 0.5 is fortuitous. This generally accepted ratio of 0.5 is the result of conditions under which the determination is made and may vary within wide limits. Review of the literature dealing with quantitative exps. shows that the ratio is usually near 0.3. indicating a marked utilization of hydrogen for the reduction of acetic acid to 2,3-butylene glycol.